Hi,
I found the below sentence in a dictionary.
His quality of life has improved dramatically since the operation.
But sometimes it seems that "His quality of life has been improved dramatically since the operation." is OK.
Can I use both of them? Yes.
Even if both are OK, is there any difference of nuance? The passive form in sentence 2 tells us that the improvement was caused by some agent, eg
"His quality of life has been improved dramatically (by new medication/by his doctor) since the operation."
Consider simpler examples.
The water has boiled.
The water has been boiled (by Mary).
Best wishes, Clive